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Recriutment of Ascophyllum Nodosum

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Recriutment of Ascophyllum Nodosum MARINE ECOLOGY PROGRESS SERIES Vol. 61: 263-272, 1990 Published March 22 Mar. Ecol. Prog. Ser. 1 l Recruitment of Ascophyllum nodosum: wave action as a source of mortality* Robert L. Vadas, Wesley A. Wright, Steven L. Miller" Departrnent of Botany & Plant Palhology, University of Maine, Orono, Maine 04469. USA ABSTRACT: The brown alga Ascophyllum nodosuni (L.) Le Jolis is a dominant rocky intertidal organlsm throughout much of the North Atlantic Ocean, yet its inabilily 10 colonize exposed or denuded shores is well recognized. Our experimental data show that wave action is a major source of mortality to recently settled zygotes. Artificially recniited zygotes consistently exhibited a Type IV survivorsh~p curve in the presence of moving water. As few as 10, bul olten only 1 reLalively low energy wave removed 85 to 99'% of recenlly setlled zygotes. increasing the setting time for attachment of zygotes (prior to distilrbance from water movemenl) had a positive effect on survival. However, survival was significantly lower at high densities, and decreased at long (24 h) setting times, probably as a result of bacleria 011 the surface of zygotes. Spatial refuges provided significant protection from gentle water movement but relatively little protectjon from waves. These data indicate that zygotes are maladapted lor attachment in moving water and suggest that water movement is the primary faclor conlrolliny recruitment and distributional patterns of A. nodosum. These and earlier observations on the long-term lack of colonizat~onof denuded shores suggest that successful recruitment is highly epjsodic on all but the most sheltered shores. Because of the widespread don~ina~~ceof A. nodosum, disturbance by waves or currents, and stochastic events may play major roles in structuring intertidal comrnunilies In the Northwest Allantic. INTRODUCTION nell 1972. Grant 1977, Vadas 1979, Denny et al. 1985). Wave energy also causes indirect mortality to juvenile Wave action is generally considered an important and adult assemblages through movemenl oI algal Iactor in the distribution and abundance of intertidal fronds, logs, rocks and debris (Connell 1961, Dayton organisms. There are, however, Iew quantitative data 1971, Velimirov 1983. Sousa 1984). Considerably less is on the specific effects of waves on patterns of species known about the effects of wave forces on juvenile abundance. Biological exposure scales describe the stages of marine organisms. Wave action, however, direct or indirect effects of waves (Ballantine 1961, clirectly reduces the density of newly recruiled barna- Dalby et al. 1978). but they do not characterize what cles (Ortega 1981). Few studies have analyzed wave aspects ol wave energy are important to the dynamics effects on recruiting or juvenile algae. Nonetheless, of a species. Attempts to quantify wave forces are such data may be essential for understanding the noteworthy (Jones & Demetropoulos 1968, Harger abundance patterns and dynamics of a species and 1970, Doty 1971), although they all have limitations community organization (Sebens & Lewis 1985). (Denny 198%). In general, little attention has been Along the northern Gulf of Maine the dominant paid to measuring abiotic forces on rocky shores inlertidal organism is the brown alga Ascophyllum (Palumbi 1984). nodosum (L.) Le Jolis (Keser et al. 1981, Topinka et al. Waves affect intertidal communities by damaging 1981, Vadas & Wright 1986). Despite its abundance on indivicluals and by displacing plants and ani~~ials, these and other North Atlantic shores, 2 striking pat- either individually or in groups (Schwenke 197 1, Con- terns are apparent: a marked decline or absence of plants (especially juveniles) on wave-exposed shores and a general lack of recruitment on all shores (Olt- * Maine Agricultural Experiment Station Publication No. manns 1889, devirville 1953, Printz 1956, Boney 1966, 1407 .* Present address: NURC-FDU, #40 Estate Castle Coakley, Baardseth 1970). Twenty years of observation at St. Croix, U.S. Virgin Islands 00820 Lamoine Maine, USA, revealed no successful coloniza- (c) Inter-Research/Printsd in F. R. Germany 264 Mar. Ecol. Prog. Ser. 61: 263-272, 1990 tion by A. nodosum clespite its abundance at this and capacity of 21 Oh,determined by drying for 48 h, boiling adjacent sites (Vadas & Wright 1986). This particl~lar in distilled watcr for 2 h to remove air, soaking for 24 h, population has noticeably declined in cover during the weighing ancl drying to a constant weight at 60°C. last 5 to 10 yr. Several other sites in Maine show similar The surlaces of some pottery chips (Expt 4) were recruitment patterns, with few exceptions (Keser & modified to s~rnulate natural surfaces and provide Larson 1984). refuges for zygotes and germlings. One half of the Although Littorina littorea (L) grazes on or disturbs unfirecl clay surface was imprinted with a nylon mesh sporelings of Asco~~hyllumnodosun) (Sundene 1973, netting while the other half was imprinted with A50 Watson & Norton 1985, Patterson '1986), cage experi- grade sandpaper. This technique created 4 mi- ments designed to exclude this herbivore have pro- crohabitats: vertical and horizontal grooves, flat sur- duced only one natural recruit since 1972 (Vadas faces, and pits. Approximately 32 squares per chip (flat u~ipubl.).Moreover, attempts to recruit A. nodosum surfaces, 5 X 4 mm) were formed by the mesh. The artificially by pouring zygotes over numerous caged depth and width of the grooves ranged from 0.1 to natural surfaces have also failed to produce significant 0.2 mm and from 0.15 to 0.2 mm, respectively. The recruitment (Vadas & Wright 1986). This apparent grooves were designated vertical (parallel to water enigma, the dominance of A. nodosum and the concur- flow) and horizontal (perpendicular to water flow); sur- rent lack of visible recruitment on these and European vival was analyzed separately in each. The number and shores, raises the question of what controls coloniza- size of the squares and characteristics of the grooves tion? varied slightly because of differential stretching of the Here we examine recruitment and early survival nylon and differences in firing and cooling tem- (524 h) of zygotes in artificial and natural wave energy peratures. erlvironments as a function of setting or attachment time, Zygotes were seeded onto pottery chjps as follows density and surface texture. In particular, weaddress the Gametes of AscophyUum nodosum are reIe,ised natur- following questions: (1)Does wave action affect recruil- ally over a 4 to 6 wk period (April to June) 111 Maine. mentantl,if so, what controlsits effect? (2)Doessetting or Eggs and sperm were obtained by separately forcing attachment time affect recruitment? (3) Are refuges release (by drying) of rnale and female receptacles in important to recruitment and early survival? Our experi- shaded trays. Gamete release occurred in 1 to 2 h on mentsdemonstrate that wave action, and water motionin mild sunny days, but took considerably longer under general, is a major source of mortality to zygotes 01 cool cloudy conditions. Gametes were collected by Ascophyllun~ nodosuzn. Longer setting times and rinsing receptacles in beakers containing cool (l0 to refuges enhance survival but only in gentle flowing 15°C) seawater. To initiate lerlilization, gametes of waters.These results suggest that zygotesof A. nodosum each sex were diluted with seawater, co~nbinedto are mr11ddaptc.d to water nioven~cr~l,and that natural produce a dark turbid broth and stirred for 30 S. In the recr11itrnr:ntis r.pisorlir, vscept perhapson vtbrysheltered first experiment 15 min was allowed for fertilization. shores and among holdfasts in dense stands. Subsequenlly a minimum of 30 min was employed. Following fertilization the zygote suspension was further diluted and gently poured over chips in shallow METHODS pans. The chips were covered by l cm of the suspen- slon and left ~~ndisturbed.The pans were shaded and Pottery chips were used as a synthetic substrate for covered witt: ,~lurnlnumfoil to prevent warming and recruitment because of their favorable attachment enhance germinatinn. After the prescribed setting qualities, water holding potential and ease of removal time, the solulion was careiully siphoned from the pans for sampling (Hanic & Pringle 1978). The clay consisted to minimint disturbance. The chips were randomized, of a mixture of Tennessee Ball Clay ancl Kaolin. Clays gently removed from the pans, subjected to wave or containing lron oxides were avoided because they may water movement ancl then placed in moist, covered be toxic. Silica, nephaline, bentonite and whiting were pans until sampled. Zygotes were regularly misted added to the mixture; silica to lower the melting point, (fine spray) with cold seawater to prevent desiccation nephaline and bentonite as suspension agents, and until censusing was complete. whiting as a colori~lgagent. Upon firing the clay lurncd Zygotes in Expts 1, 2, and 3 were counted by first a flat white provid~nga reflective rather than a heat- estimating densities of entire chips as light, medium absorbing surface. Clay was rolled out in 1 cm thick and heavy, and then by random subsampling using slabs, cut into 5 cm X 5 cm squares and allowed to dry eyepiece grids. In Expt 4, zygotes were co~intedon 18 to a leathery consistency. Holes (0.5 cnl) were drilled squares (flats) on light density chips (< 20 per flat).On through the centers of the squares which were then medium density chips (21 to 200 per flat), zygotes were fired at 983°C (Cone 8).The chjps had a water-holding counted on G randomly selected llat squares. For high Vadas et al.: Recruitment of an ~ntertidalalga density chips (>200 per flat), 6 randomly selected flats lnvolved in the low survival at long setting tirnes in were subsampled using an eyepiece grid. The number pilot studies and Expt 2. Zygotes were subjected to 9 of zygotes in randomly selected horizontal and vertical setting intervals in 2 series: (a) 0.5, 1, 2, 3 and 4 h and grooves was estimated and placed into 7 categories: 0, (b) 6, 12, 16, and 24 h.
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